The present invention relates to a method and a system enabling the formation of hydrates to be prevented in a fluid containing at least one gaseous phase and one aqueous phase whilst it is being transferred from one location to another under conditions that may vary and induce the formation of hydrates.
The present invention is of particular advantage when applied as a means of preventing the formation of hydrates of light hydrocarbons, such as the hydrates of natural gas, petroleum gas or other gases within a fluid.
These hydrates may form where water is to be found in the presence of light hydrocarbons, either in a gaseous phase or dissolved in a liquid phase, such as a liquid hydrocarbon, and in particular if the temperature of the mixture falls below a critical temperature (thermodynamic temperature at which hydrates form), which is dependent on the composition of the gases and the pressure. These latter are inclusion compounds which result when water molecules assemble to form cages in which the molecules of light hydrocarbons such as methane, ethane, propane or isobutane are trapped. Some of the acid gases present in natural gas, such as carbon dioxide or hydrogen sulphide, may also form hydrates in the presence of water.
It will be recalled that in the case of a multi-phase fluid containing an aqueous phase and a gaseous phase, when the temperature and pressure vary, it is possible to determine a range within which hydrates form on the basis of the composition of the fluid, this range being limited by a pressure curve and the temperature of a given composition.
In order to reduce the cost of producing crude oil and gas, both in terms of capital investment and development, one of the options considered, particularly in offshore production, is to reduce or even do away with all processing of crude oil or gas to be transported from the reservoir to the coast and in particular to leave all or some of the water in the fluid to be transported. The petroleum effluent is then transferred by pipeline in the form of a multi-phase fluid to a processing platform. This approach is of particular advantage if the offshore production site is located in a zone that is difficult to access. Nevertheless, it does have a major disadvantage in that there is a risk of hydrate formation due to the presence of water and the inherent conditions, particularly the surrounding environment.
In practice, petroleum effluent containing a gaseous phase and possibly a liquid phase can be made up of a natural gas, a condensate gas or an associated gas, for example, mixed with the crude oil. They are generally saturated with water and in some cases may even contain free water.
When transporting production effluent along the seabed from a subsea natural gas or oil and gas reservoir containing water, the prevailing temperature may be quite low, in the region of a few degrees, and the temperature of the effluent changes as it passes along the pipe and may reach relatively low levels at which the conditions are conducive to hydrate formation.
These hydrates cause the transportation pipelines to fill up and block, which eventually prevents the oil or gas from passing through and leads to extremely serious consequences: production downtime can often be quite protracted since it is very difficult to break hydrates down once they have formed, which then incurs substantial financial losses.
Under the requisite thermodynamic conditions, other parameters may also induce hydrate formation, such as changes in the shape of the pipe, for example elbows, as well as variations in the flow speed and any occurrence of turbulence within the flow.
Conditions conducive to hydrate formation may also arise during a production stoppage or as a result of changes in the flow rate caused on the production side.
The present invention has the advantage of overcoming the potential consequences arising from variations in the effluent flow rate during production in the majority of petroleum production and transportation applications.
These variations in flow rate can cause major changes in the pressure and temperature profiles along the pipe, creating detrimental conditions in certain zones of the pipeline which could lead to hydrate formation.
This being the case, the present invention makes allowance for changes in the pressure and temperature profiles associated with variations in the flow rate as a means of controlling the conditions prevailing across the full length of pipeline and bringing and/or retaining the fluid outside the range within which hydrates form.
The same applies in the case of natural gas, which is also susceptible to hydrate formation under certain conditions.
Conditions conducive to hydrate formation can also occur in the same way onshore if pipes are buried close to the surface and the ambient air temperature is quite low, for example, especially in northern zones such as the arctic zones.
Various methods have been described in the prior art as ways of avoiding these drawbacks.
One method aimed at removing the water can be carried out on a platform located at the surface close to the reservoir so that the effluent, which is hot initially, can be treated before the effluent has been cooled by the sea water to the point at which hydrates form. However, this solution means that the effluent has to be brought up to the surface before it is transferred to a main processing platform and an intermediate processing platform has to be provided.
Another approach is to fit the transportation pipeline with insulating means or equip it with heating means, such as the device described in patent U.S. Pat. No. 5,241,147, so as to prevent the fluids from cooling too rapidly whilst being transported. However, such devices are costly, all the more so as they generally have to be provided to cope with the most difficult situations encountered.
Other methods involve the use of radiation. For example, patent HU 186511 discloses a method of emitting an electromagnetic wave, whose frequency values and propagation modes are selected for their ability to melt any hydrates that have formed.
In patent SU 442287, an ultra-sound wave is used to break up the hydrate crystals and release the trapped gas.
A technique such as this is costly and often harmful to the environment.